So you think that monkey bars are for kindergarteners, and that rope climbs belong in second-period gym classes? Think again. Tough Mudder knows that the easiest looking obstacles can cause the biggest trouble. Each course is designed to test your physical fitness and your mental strength — that means not only pushing your body to the max, but conquering your worst fears: heights, frigid water, electric shocks, fire, you name it. We talked to Tough Mudder’s course design manager Nolan Kombol, who broke down the obstacles that’ll most likely trip you up. Study up — and never let your guard down. Tough Mudders 20 Most Badass Obstacles >>>

1. Arctic Enema

What it tests: Mental gritWhy it’s harder than it seems: Plenty of athletes use ice baths for recovery, so splashing your way through some chilly water should be a cakewalk, right? Wrong. The frigid temperature makes your body react in unexpected ways — loss of breath, contracted muscles, impaired motor functions (or as the Tough Mudder course designers describe it, “Like eating too much ice cream and getting kicked in the balls at the same time.”) Instead of getting out of the icy water as quickly as possible, then, many racers’ bodies freeze up—literally.Completion Rate: Kombol estimates that 100% of Mudders complete this task — but only 20% do so without panicking.

2. Twinkle Toes

What it tests: BalanceWhy it’s harder than it seems: You’re expected to traverse this narrow wooden beam over an icy pit, racing against other competitors balancing parallel to you. You can weigh 220 pounds and be able to bench the moon, but without balance and concentration, you’ll find yourself ass-first in the water. Twinkle Toes “tends to be an equalizer [between men and women],” says Kombol of the balance-beam act.Completion rate: Roughly 40%, says Kombol.

3. Boa Constrictor

What it tests: Fear of small spaces and upper-body strengthWhy it’s harder than it seems: In this obstacle, you crawl downhill through a black tube into freezing water, then uphill out the other side. You anticipate the tight, claustrophobia-inducing space, but as you inch your way higher, a steep incline awaits. “The angle of exit makes pushing your way through very difficult,” says Kombol. An extra push of upper body strength will get you though—as long as you don’t lose it first.Completion Rate: Kombol estimates that only 40% are able to make it out without any help.

4. Hangin’ Tough

What it tests: Agility, hand strengthWhy it’s harder than it seems: Here you’ll swing your way across a pit of icy water, grabbing rings greased in oil and hanging four to six feet apart. (Buildup from mud and ice make the rings extra slippery too.) And while you would think upper body strength is key here, strong hand muscles are actually what will give you an advantage, helping you establish a rhythm as you swing across.Completion rate: Roughly 30%, says Kombol.

5. Dong Dangler

What it tests: Upper body strengthWhy it’s harder than it seems: You can see point B from point A, but inching your way 70 feet across a freezing lake on a metal cable is tougher than it looks. (Pro tip: Hanging upside down by hooking your ankles together will give you leverage and extra hold.) “This requires an enormous amount of shoulder strength and the ability to not panic when you’re in this position,” says Kombol.Completion rate: Roughly 65%, says Kombol.

6. Berlin Walls

What it tests: Teamwork, upper body strengthWhy it’s harder than it seems: This series of three 12-foot wooden walls is impossible to do alone and designed with one focus in mind: teamwork. Sure, the climb itself will be tough, but don’t underestimate the toll of assisting your fellow Mudders up after you. Reveling in your successful ascent doesn’t mean you’re done, either. Think carefully about your exit, which can be just as challenging.Completion rate: Kombol says helping four people over the wall reduces your own ability to successfully complete the obstacle by about 50%.

7. Island Hopping

What it tests: BalanceWhy it’s harder than it seems: While it might look like a simple task of leap frog, hopping across unstable wooden boxes suspended by loose ropes in an icy pond proves nearly impossible. Boxes are covered with water from previous Mudders who’ve taken the plunge, making them extra slippery. The trick is to take your time and focus on conquering one island at a time; otherwise, you’ll fall into the lake and serve a time penalty.Completion rate: Roughly 40%, says Kombol.

8. Cliff Hanger

What it tests: Teamwork, agilityWhy it’s harder than it looks: You’ve scaled tons of hills during your training, so how bad can this slippery heap of mud be? Well, this baby’s built at a 45-degree angle—and bigger than anything you could have expected. As the Tough Mudder website warns, “If you want to train for Cliff Hanger, you should find the biggest hill near your house, measure it, then drive until you get to a hill twice as steep.” Work your Mudder camaraderie here—form a chain, give a push, and always look back to lend a hand.Completion rate: Roughly 80% complete this obstacle, says Kombol, but it can be done only with the help of other Mudders.

9. Dark Lightning

What it tests: Fear, agilityWhy it’s harder than it looks: You’re not afraid of the dark, but a crawl through this cave offers plenty of surprises— like a foot in the face from a Mudder in front of you, or a zap from an electric shock. As one Mudder described it on MudderNation.com [http://muddernation.com/profiles/blogs/new-obstacle-dark-lightning], “I was as close as a man could get to the ground without getting it pregnant, and I still got zapped five times working my way through.”Completion rate: Kombol says that all Mudders complete this task (once you go in, there’s no way out but the other side) — but only a few do so without panicking.

10. Funky Monkey

What it tests: Agility, hand strengthWhy it’s harder than it looks: You all conquered the jungle gym monkey bars in the 1st grade, so why does Funky Monkey — which is pretty similar — trip up 3/4 of the Mudders who try to cross it? Like Hangin’ Tough, the ice and mud buildup from those who’ve come before you is no match for numb fingers. Swing your body using your arm strength, and grab for the rings with a firm grip. But be warned: try as you might, you’ll likely slip into the icy water below.Completion rate: Roughly 25%, says Kombol.